U.S. Pat. No. 4,760,420, entitled, "Shutter Mechanism For A Camera" by Lam, discloses a camera shutter mechanism having overlapping plates which are pivotally mounted and connected by a pin sliding in slots in the plates so that the plates move in unison. Springs bias the plates to an overlapping position, covering a hole in the camera body which leads to a lens. One of the plates is struck by a lever which urges the plates against springs causing them to pivot away, until the lever strikes a stop at which point the plates still overlap sufficiently to cover the hole. After the lever strikes the stop the plates continue to move under their own momentum to open up an aperture bound by the edges.
A magnetic control plate with pawls is attracted towards the overlapping plates by an electromagnet so that the pawls engage racks on the plates to arrest the plates at a particular aperture setting. A light metering device or optical sensor is used to determine the position of the plates and thereby to control energization of the electromagnet. The electromagnet must be energized slightly before the desired aperture is reached to allow time for the magnetic control plate to move into the engaged position. When the electromagnet is switched off, the overlapping plates are released which closes the aperture under the action of the springs.
An improvement in the type of shutter shown in the 4,760,420 patent is disclosed in U.S. Pat. No. 5,432,576, entitled, "Aperture Mechanism Having A Blade Positionable In An Open Position By A Hard Stop" by SanGregory et al. In that patent hard stops are used to define the limits of travel for the aperture plates. The hard stops are positioned in their stopping position by activating an electromagnet that in turn projects a magnet (piston) into the path of the blade. To maintain the piston in that position the electromagnet remains energized. To retract the piston the current flowing through the electromagnet is reversed.
The present improvement is directed to minimizing the current draw that is needed to actuate and to hold the piston in place as the current is drawn, generally, from a battery.